As a cage for holding a rolling element of a rolling bearing, a metal cage made of steel or brass may be used. The metal cage is excellent in strength but results in high cost when being processed into a complicated shape disadvantageously. Further, the raw material thereof has a large specific gravity to result in a large mass. Accordingly, such a metal cage is unfavorable for bearings for high-speed rotation applications or transportation equipment applications, which require reduced weight, disadvantageously.
In contrast, cages made of resin have been known. Such cages can be molded readily and achieve reduced weight. Further, there have been proposed cages each made of a resin fiber-reinforced by incorporating a fiber into the resin (for example, see Japanese Patent Laying-Open No. 2005-127493 (Patent Literature 1), Japanese Patent Laying-Open No. 2005-140269 (Patent Literature 2), Japanese Patent Laying-Open No. 2005-163997 (Patent Literature 3), and Japanese Patent Laying-Open No. 2007-78118 (Patent Literature 4)). These make it possible to provide a cage that can be processed into a desired shape at low cost and achieves reduced weight while securing a certain level of strength.
For applications requiring a higher strength while achieving reduced weight, it has been also proposed that a cage manufactured by semi-molten molding of a magnesium alloy is applicable. Accordingly, there can be provided a cage suitable for high-speed rotation because of its excellent moldability, high strength, and light weight (for example, see Japanese Patent Laying-Open No. 2000-213544 (Patent Literature 5)).
Further, there are an iron plate cage, and a fiber-reinforced resin cage employing an engineering plastic or a super engineering plastic as its matrix. Examples of the engineering plastic and the super engineering plastic include a nylon (polyamide; PA) resin, a polyphenylene sulfide (PPS) resin, a polyetheretherketone (PEEK) resin, and the like. (For example, see Japanese Patent Laying-Open No. 64-79419 (Patent Literature 6)).
Because the iron plate cage and the brass cage are made of metals, they are excellent in strength. However, since the iron plate cage is mainly manufactured by means of press processing, it is difficult to manufacture a cage having a three-dimensionally complicated shape. In addition, assembly thereof is also complicated because two components need to be connected to each other (for example, see Japanese Patent Laying-Open No. 8-103840 (Patent Literature 7)). When economically manufacturing the brass cage, the brass cage is usually formed into a simple machined shape. Hence, both the iron plate cage and the brass cage have restrictions in terms of their shapes that can be industrially manufactured. These cages are larger in specific gravity and therefore heavier than a resin cage. Hence, they are not much suitable for applications under high-speed rotation, which require light weight (for example, see Patent Literature 1). Accordingly, resin cages have begun to be used for rolling bearings for main shafts of machine tools required to attain high-speed rotation, for example. Further, under an intermittent operation, energy is lost in proportion to the mass of a cage during acceleration and deceleration. Hence, it is desired that the cage has a light weight also when it is used for a business machine and a moving medium such as a vehicle, both of which strongly require energy saving.